Myogenic cells grown in culture retain many of their differentiated, cell surface associated properties. Using our recently developed methods for systematically altering the myoblast lipid composition, we have obtained evidence suggesting an intimate, direct lipid involvement in myoblast fusion. From these and other observations we have developed a working hypothesis for myoblast fusion in which fusion is a lipid mediated event whose rate is modulated by the physical state of the lipids. The overall objective of this proposal is to test and develop this hypothesis. In the upcoming grant year, we propose to determine the distribution of lipids among the membrane surfaces. We will also explore the changes in membrane fluidity that are alleged to correlate with differentiation and the onset of fusion competence.